The present invention relates to radio transceivers which operate to both transmit and receive radio signals. In particular, but not exclusively, the present invention relates to radio transceivers, which are provided with a means to isolate signals transmitted by a transmitter of the transceiver system and received by a receiver of the transceiver system.
In modem radio communications equipment it is often a requirement to both transmit and receive radio signals. Such transmitted and received radio signals may be on different frequencies separated by a guard band. As such, in order to ensure that signals transmitted from a transmit antenna of the transceiver do not corrupt and distort signals received on a receive antenna, it is necessary to provide isolation between the transmitter and receiver of the transceiver system.
In broadband Code Multiple Access systems (CDMA), the transmit and receive signals have a relatively wide bandwidth. As such, the bandwidth of the transmit and receive signals may be comparable to the guard band separating the transmit and receive frequency bands.
In known systems, a duplexing filter is provided which is coupled to the output of the transmitter and the input of the receiver in order to effect the desired frequency separation between transmitter and receiver.
The purpose of the duplexing filter is to provide isolation between the Transmit (Tx) and the Receive (Rx) ports whilst connecting the Tx and the Rx to a common antenna. This is achieved by filtering the appropriate Rx and Tx frequencies prior to combining them at the antenna port. A typical duplexing filter achieves 30 to 40 decibels (dB) isolation between the Rx and the Tx ports. Since the attenuation is provided by filtering, each duplexing filter is dedicated to a specific band of Rx/Tx frequencies.
It is well known in the art of radio transceiver systems that dual or multi-mode handsets, operating in two or more frequency bands, use a separate duplexing filter for each frequency band, with the appropriate band being selected by a switch. Since the duplexing filters are relatively large components, this approach becomes less attractive for multi-mode operation due to lack of space in the handset.
A truly multi-mode terminal must have a flexible architecture which covers arbitrary standards and frequencies. For such a terminal, a key device is a duplexer, which provides Rx/Tx isolation without the need for filtering which is dedicated to specific frequencies.
A further disadvantage with known duplexing filter based transceiver systems is that an inherent insertion loss associated with the connection of the duplexing filter to the receiver, causes a degradation in the receiver noise figure and furthermore reduces the power with which the transmitted signal may be transmitted. Another disadvantage with duplexing filters is that impairment is caused to the received and transmitted signals as a result of in-band amplitude and group delay ripple effects caused by the duplexing filter. This, in turn leads to loss of capacity in a radio system such as a CDMA system. The impairment is more significant in broadband systems such as broadband CDMA, in which the bandwidth of signals is comparable to the receiver and transmitter frequency separations.
The aforementioned disadvantages represent a technical problem which is addressed by the present invention.